In a hydraulic oil traveling drive apparatus for a vehicle which is driven by connecting the output torque of a plurality of hydraulic oil motors by means of a clutch; the output torque, the vehicle speed, and the like are conventionally controlled by connecting or disconnecting the clutch when the vehicle speed becomes a predetermined value.
FIG. 7 is a control circuit of a conventional control apparatus for a plurality of hydraulic oil motors and a clutch. The control circuit is constituted by the hydraulic oil pump 50, the first hydraulic oil motor 51, the second hydraulic oil motor 52, the clutch 53, a hydraulic oil pump 54, and the like. The first hydraulic oil motor 51 and the second hydraulic oil motor 52 are connected to the hydraulic oil pump 50 in parallel, and are driven by pressurized oil discharged by the hydraulic oil pump 50. The first motor gear 51b is fixed to the first output shaft 51a of the first hydraulic oil motor 51, and the first motor gear 51b is engaged with the gear 55a, which is fixed to the drive shaft 55 for driving the vehicle. The output torque of the first hydraulic motor 51 is always transmitted to the drive shaft 55 via the first motor gear 51b and the drive shaft gear 55a.
The clutch 53 is provided in the output shaft 52a of the second hydraulic motor 52. The second motor gear 53b, for the second hydraulic motor 52, is fixed to the second output shaft 53a of the clutch 53, and the second motor gear 53b is engaged with the gear 55a for the drive shaft 55. When the clutch 53 is engaged, the output torque of the second hydraulic motor 52 is transmitted to the drive shaft 55 for driving the vehicle via the clutch 53, the second motor gear 53b, and the drive shaft gear 55a. A servo valve is attached to each of the first and second hydraulic motors 51 and 52, and the discharge volume (cc/rev) of each of the first and second hydraulic motors 51 and 52 is controlled in accordance with a pressure signal from a high and low valve (hereinafter, referred to as an HL valve, not shown) for selecting between a high speed and a low speed.
The hydraulic oil pump 54 is connected to the drive shaft gear 55a via the pump gear 54a and the second motor gear 53b. The shuttle valve 54c, positioned in the conduit 54e, and the throttle 54b are connected in parallel between the Fp port and the Rp port of the hydraulic oil pump 54. When a rotating speed of the drive shaft 55 (corresponding to the vehicle speed) becomes equal to or greater than a predetermined speed value, the discharge pressure of the hydraulic oil pump 54 reaches a predetermined pressure value which disengages the clutch 53 via the conduit 54d. When the rotating speed of the drive shaft 55 is less than the predetermined speed value, the discharge pressure of the hydraulic oil pump 54 does not reach the predetermined pressure value, so that the clutch 53 is still engaged. Further, the hydraulic pump 50 and the first hydraulic motor 51 are always connected to each other; however, the communicating and shutting valve 56 is interposed in the conduit 54f between the hydraulic pump 50 and the second hydraulic motor 52, so that in a first position of the valve 56 the second hydraulic motor 52 is isolated from the hydraulic pump 50, and in a second position of the valve 56 the second hydraulic motor 52 is connected to the hydraulic pump 50.
A function of the communicating and shutting valve 56 will be described. When a tilt rotation amount is present in the second hydraulic motor 52, a speed changing shock is generated by a disengaging of the clutch 53; therefore, the tilt rotation amount of the second hydraulic oil motor 52 is set to be zero (hereinafter, referred to as a zero tilt rotation) before disengaging the clutch 53. However, since there is no means provided for holding the zero tilt rotation position for the second hydraulic motor 52, when even just a little tilt rotation amount is provided at the second hydraulic motor 52, due to a sudden speed change or the like after setting the second hydraulic motor 52 to be at the zero tilt rotation (refer to the broken line S5 in FIG. 9), the second hydraulic motor 52 races and a load slip of the first hydraulic motor 51 is generated. In order to prevent the load slip, after the second hydraulic oil motor 52 is set to be at the zero tilt rotation, the conduit 54f, which connects the hydraulic pump 50 to the second hydraulic motor 52, is shut by the communicating and shutting valve 56 at the same time as the clutch 53 is disengaged.
An operation of the conventional control circuit will be described below. FIG. 8 shows the relation between the hydraulic motor discharge volume (cc/rev) and the vehicle speed (km/h) in the low speed range Lo. The curve S1 shows the relation for the first hydraulic motor 51, and the curve S2 shows the relation for the second hydraulic motor 52. For example, since the wheel loader is structured such that the discharge pressure of the hydraulic pump 54 does not reach a predetermined pressure value until the vehicle speed is 12 km/h, the clutch 53 is in the engaged state. Accordingly, the wheel loader is driven by a large drive force, corresponding to the sum (S1+S2) of the discharge volumes of the first hydraulic motor 51 and the second hydraulic motor 52. In this case, when it is set to the low range Lo by the HL valve (not shown), the discharge volumes of the first hydraulic motor 51 and the second hydraulic motor 52 are limited, and the maximum vehicle speed is increased only to 12 km/h.
FIG. 9 shows the relation between the hydraulic motor discharge volume (cc/rev) and the vehicle speed (km/h) in the high speed range Hi. The curve S3 shows the relation for the first hydraulic motor 51, and the curve S4 shows the relation for the second hydraulic motor 52. In this case, since the discharge pressure of the hydraulic pump 54 does not reach the predetermined pressure value until the vehicle speed is 15 km/h, the clutch 53 is in the engaged state. When the vehicle speed is near 15 km/h, the discharge volume S4 of the second hydraulic motor 52 approaches zero. Thus, when it is set to the high speed range Hi by the HL valve (not shown), the limit of the discharge volume of the first hydraulic motor 51 is canceled, and the vehicle speed is increased to 30 km/h or more by only the discharge volume S3 of the first hydraulic motor 51. Further, when the vehicle speed is 15 km/h or more, the discharge pressure of the hydraulic pump 54 reaches the predetermined pressure value, and the clutch 53 is disengaged.
In the conventional hydraulic oil traveling drive apparatus described above, since the communicating and shutting valve 56 must be a large size so that the total discharge amount of the second hydraulic motor 52 can pass therethrough, there is a problem in that the cost for the piping and for the communicating and shutting valve 56 is great, and a large space is necessary. Further, since it is necessary to shut the communicating and shutting valve 56 at the same time as disengaging the clutch 53, a speed changing shock and the like are generated when this delicate timing is not satisfied. Further, in the case of fixing each of the first and second hydraulic motors 51 and 52 with a large tilt rotation amount and increasing the brake torque so as to reduce the vehicle speed, there is a case wherein the vehicle overruns when the fixed tilt rotation amount is still large and is suddenly canceled. Accordingly, it is necessary to hold the tilt rotation amount, increased during the speed reduction, until a cancel instruction is outputted.